Scientists used crowd-sourced data from the eBird platform to model how the distributions of 21 migratory bird species change throughout the year and to determine how these distributions overlap with protected areas and projected changes in climate and land-use.

They found that the conversion of natural forest to agricultural and/or urban land uses will eliminate habitat in the near term, particularly in the birds’ tropical overwintering areas, while rising temperatures and diverse changes in rainfall would affect all points of the species’ ranges over the longer term.

The researchers suggest that enhanced management of less strictly protected reserves in the birds’ wintering grounds could mitigate the near-term forest habitat losses where the birds spend nearly 60% of their time and perhaps give species time to respond to longer-term climate-related changes in food and habitat availability.

A multi-national research team combined four diverse data sets to understand where migratory birds are most vulnerable to human activity and to identify priorities for protecting long-distance migrants in the future. They analyzed millions of crowd-sourced bird observations to assess the geographic distributions of 21 species of birds that use primarily forest habitat and migrate between North and Central America each year.

By comparing the millions of point locations of these species with land-use change and climate projections, they established that bird populations were threatened most in the coming years by the loss of forest habitat in their winter non-breeding areas. In the long term, projected warming temperatures and declining rainfall may reduce the quality of their breeding and non-breeding habitat, respectively.

Bringing citizens into science
The researchers compiled bird observation data that had been collected and entered into the eBird database by citizen scientists across the birds’ temperate forest breeding areas, in their tropical forest non-breeding areas, and along their migration routes between the two zones.

For this study, the research team compiled crowd-sourced location data for 21 forest bird species that breed in North America and migrate to Central America each year to spend the winter. The species included 12 warblers, 3 flycatchers, and 2 vireos, as well as wood thrush, indigo bunting, rose-breasted grosbeak, and orchard oriole.

A chestnut-sided warbler (Setophaga pensylvanica), one of 21 migratory bird species in the study. All are associated with forest habitats. Photo credit: Francesco Veronesi, CC 2.0

The 2014 eBird Reference Dataset (Munson et al., 2015) for these 21 species included observations from 5,763,369 complete observer checklists (all the birds a given observer can detect, regardless of ability). The observations were recorded between 2004 and 2014 at 880,310 unique locations in the Western Hemisphere north of the Equator.

The aim for eBird is to “maximize the utility and accessibility of the vast numbers of bird observations made each year by recreational and professional bird watchers.”

Birdwatchers love to record what they see. If you are one of them, eBird is an online checklist program that enables you to log the birds you see in the field, review them later on, and compare your checklists to those from other people and places. It also makes the checklist data available to scientists.

Recognizing the large numbers of people watching birds and the enthusiasm birders have for recording and sharing their sightings, the Cornell Lab of Ornithology and National Audubon Society launched eBird in 2002 to see how data collected by citizen scientists could contribute to research and conservation efforts.

An eBird map of sightings of the chestnut-sided warbler, which winters in Central America and breeds in North America. Darker purple pixels are those where the species is seen more frequently. This map presents data for all weeks of the year. Image credit: eBird

The database increases when birders log onto the site, enter when and where they went birding and the effort they put into it, and check off all the suggested birds they saw and heard during their outing. eBird’s automated data quality filters then review the checklist before it enters the community database. Local experts review any records that the filters flag as unusual for the time and place.

Tens of thousands of users submit these observations or view results via interactive online queries into the curated eBird database. The site allows users to maintain personal bird counts online and to view the shared data through interactive maps and charts in English, Spanish, and French.

eBird compiles all the information to show species presence at a given time and place and to determine bird abundance through the checklist data.

Map of eBird reported sightings of chestnut-sided warbler in Central America, where this and the other 20 study species spend over 50% of their year. This species is in the region from October through May. The data here are summarized into pixels, with darker purple cells reflecting higher numbers of records. Image credit: eBird

Crowd-sourced data have some error: for eBird, some users log the presence of a species but cannot or do not add an accurate number of birds they saw, and of course some users can identify the birds far better than others. eBird and other crowd-sourced databases gain their strength from numbers; with many contributions, the site’s algorithms can estimate relative abundance over time and space.

Layering local, regional, and global-scale datasets
The research team used the eBird data to document weekly patterns of occupancy and relative abundance for the 21 study species within 3.3 x 3.3 km pixels across their ranges to model the probabilities of where birds were every week of the year.

They summarized the patterns of the birds’ distributions to estimate the number of species in each pixel, the total area occupied by a species during each week of the year, and proportion of each year that a species is present in each of the pixels.

Maps of (a) species richness and (b) duration summarized at a 3.3 x 3.3 km spatial resolution for the 21 migratory bird species in the study. Duration is defined as the percent of the annual cycle the 21 species occur on average within a given geographic region. Both summaries are based on weekly estimates of occurrence for the 21 species. Image credit: La Sort et al. (2017)

The paper’s lead author, Dr. Frank La Sorte of Cornell University’s Lab of Ornithology, said in an email to Mongabay-Wildtech, “With eBird, a major strength is that the data encompasses the full annual cycle, providing information on where birds occur during the breeding and non-breeding seasons and during migration. This full annual cycle perspective is essential to advance our understanding of the ecology of migratory bird populations and how these populations are being impacted by global change and other human activities such as light pollution.”

The researchers combined these local, crowd-sourced bird observation data with continental-scale data on changes in land use and climate to estimate how these changes will affect the areas where birds occur.

They examined land use maps and three land-use change scenarios using a land-systems change model called CLUMondo that combines land use, production, urbanization, and regulation alternatives. They applied the scenarios to determine the likely land use situation during each week in each pixel of the study area, projecting how and where habitat loss will most likely occur over time.

They also examined how temperature and precipitation are projected to change during the current century using 30 atmospheric-ocean general circulation models (AOGCMs) from the 2013 Intergovernmental Panel on Climate Change. They used the period 2006-2015 to define “current” reference conditions and calculated temperature and rainfall in 2091-2100 as expected future conditions.

The team generated weekly estimates of both land cover and climate to match the bird observation data to project where and how conditions will affect bird populations. They ran dozens of scenarios to determine which combinations of factors would most affect the study species in both their breeding and non-breeding areas.

A great crested flycatcher, another of the study species. Photo credit: Andrea Westmoreland, CC 2.0

“This is really the first study to measure the combined impact of climate change and land-use change over a bird’s full annual cycle,” said La Sorte in a media release. “Typically, studies tend to focus on the breeding season. If you do that, you’re missing the real story which is inherently dynamic and complex.”

The researchers also calculated the percentage of pixels for each species and week that were within protected areas (PAs) to understand the protection status for each species over time.

Near-term forest loss and longer-term climatic change
The authors found the 21 study species spend nearly 60% of their time in Central America. From October through March, the birds are recorded there within a much smaller portion of their range than during the summer breeding season, when they are recorded in lower densities across much of North America.

“Our findings indicate that land-use change on their wintering grounds in Central America may be the most pronounced threat for these birds over the next few decades,” said La Sorte in a media release. “That means more individuals of more species are likely to be exposed over a longer period of time to habitat loss as people continue to convert forests to cropland or grassland.”

An eBird bar chart of the monthly records of occurrence of a suite of warbler species. Most in this group are seasonal migrants that arrive in autumn, spend the winter, and leave the region in spring to breed in North America. Birds with larger bars are recorded more frequently. Image credit: eBird

Over the longer term, expected decreases in rainfall and warming temperatures may reduce available habitat and food on wintering grounds even further, threatening the birds’ ability to survive. Longer-term warming may also alter habitats and food availability along the birds’ migration routes and in their summer breeding areas.

With habitat loss expected to lead to precipitous population declines before climate change does, the authors of the current study recommend that private conservation organizations work with public land agencies in Central American countries to protect more forested land from development and to find ways to better manage the public lands that already exist, through sustainable forestry projects and other activities that include local communities. PAs currently cover a very small portion of the species’ full geographic distributions, and most of this area has low protection status.

By pulling together global-scale climate data, regional-scale land-use change projections, and local-scale bird occurrences, the scientists created weekly estimates of the conditions birds are expected to face wherever they are in their annual cycle over the next century.

The authors explain that better understanding of how the birds’ presence overlaps with these changing conditions throughout the year will allow the conservation community to assess the broad-scale susceptibility of migratory bird populations to global change.

La Sorte added, “A significant advantage of eBird and other citizen-science projects is that they are more flexible or less structured, allowing observers to compile a broader spectrum of information, which can be used to inform conservation and research in new and innovative ways.”

A yellow-throated vireo (Vireo flavifrons), another of the migrants in this study. Photo credit: Dan Pancamo, CC 2.0